Cerium dioxide (CeO2) based materials have been studied for use in various applications including 1) fast ion conductors; 2) oxygen storage capacitors; 3) catalysts; 4) UV blockers; and 5) polishing materials. Pure and doped CeO2 exhibits the cubic fluorite structure, similar to ZrO2. Doping CeO2 with lanthanide series elements (e.g. Gd3+) results in the formation of oxygen vacancies ([Gd3+]=2[Vooo]), and a high ionic conductivity, σi. In particular, Ce0.9Sm0.1O1.95 exhibits a σ1=0.025 (Ω*cm)−1 at 600° C., which is more than five times that of ZrO2 based materials. As such Ce0.9Sm0.1O1.95 is an attractive choice for use as a low temperature electrolyte and as an anode component in solid oxide fuel cells (SOFC).
Ceria particles can also be used as catalysts, such as three-way catalysts to purify exhaust gases, such as for automobiles. This application requires a high oxygen storage content (OSC). In order to improve the OSC, the ceria may be doped with lanthanide elements. The use of high surface area, nanocrystalline powder could benefit all of these applications.
Typically, processes for preparing nanocrystalline CeO2 involve simple oxidation of Ce metal clusters to form CeO2, or solution processes that take advantage of the small solubility product of Ce(OH)3(10−23). In addition, such processes involve reaction temperatures of 100° C. or higher. This results in larger particle sizes and lower surface area of the crystals. The particle size is inversely related to the specific surface area (“SSA”).
An example process is found in, U.S. Pat. No. 5,017,352 which discloses ceria having a SSA of at least 85±5 m2/g. The ceria particles are made from the hydrolization of cerium (IV) nitrate solution in an acidic medium and followed by calcining the washed and dried precipitate in the temperature range of 300° to 600° C. for a period of 30 minutes to ten hours. This basic process can also be used to produce ceria having a SSA of at least 130 m2/g as disclosed in U.S. Pat. No. 5,080,877. The ceria is formed by reacting an aqueous solution of cerium (IV) salt with an aqueous solution of sulfate ions to precipitate a basic ceric sulfate, washing the precipitate with ammonia and then calcined in a furnace at 400° C. for 6 hours.
It is also possible to generate single crystal grains ranging in size from 10 to 80 nm of cerium oxide that have a uniform particle size and shape. This is disclosed in U.S. Pat. No. 5,938,837 as being accomplished by mixing cerous nitrate with a base to keep the pH from 5 to 10 and then rapidly heating the mixture to 70° to 100° C. and maintaining the mixture at that temperature from about 30 minutes to 10 hours.
U.S. Pat. No. 4,786,325 discloses a method for the production of a solid solution of cerium oxide and a lanthanide series metal. This is achieved by combining a cerium salt, a basic solution, and a lanthanide salt. The mixture is reacted at either 10–25° C. or 40–95° C., filtered, dried, and calcinated at 600 to 1200° C. for a period of time of 30 minutes to 10 hours. The particles are ground so that their mean particle size is from 0.5 to 1.5 μm and the resulting SSA is from 2 to 10 m2/g.
U.S. Pat. No. 5,712,218 discloses a method for producing a solid solution of cerium/zirconium mixed oxides that optionally can include yttrium. The method involves mixing stoichiometric amounts of soluble compounds of cerium, zirconium and optionally yttrium, heating the mixture to at least 100° C., and filtering out the product. Optionally the product can be further calcinated at between 200° to 1000° C. However, it is disclosed that the calcinations process will reduce the surface area of the solid solution. The SSA of the uncalcinated solid solution can reach over 150 m2/g.